Division S-4- Soil Fertility and Plant Nutrition

Nitrogen recommendations for Upland cotton (Gosspium hirsutum L.) in the western USA are based on spring soil NO³--N tests. In-season monitoring of plant N status is another approach. Our primary objective was to test spectral reflectance and chlorophyll meter measurements as in-season N decision aids for irrigated cotton, and to compute these with soil test-based N management. The secondary objective was to determine the fate of 15N as affected by N management irrigation modes. Urea ammonium nitrate was applied with low energy precision (LEPA) center-pivot, surface drip, and subsurface drip irrigation. Microplots received 3 atom% 15N. Soil test N application was based on 0- to 60-cm soil NO³-N and 1400 kg lint hn- 1 expected yield. Thirty-four kilograms of N per hectare was applied when green vegetative index (GVI) or chlorophyll meter readings relative to well-fertilized plot were <0.95. Lint yield responded to N at Lubbock in 2000 and 2001, but not at Ropesville. Nitrogen applied with in-season monitoring in 2000 at both sites was 34 to 101 kg N hu- 1 less than soil test N application of 134 kg ha- I, with similar yields. In Lubbock. 2001 lint yields were near the expected yield, and in three of four cases, N applications with ill-season monitoring equaled soil test N applications of 101 kg hn-1. Nitrogen-15 recovery in plants ranged from 19 to 38%, and was affected by N management in two of three site-years, but not by irrigation. This study indicates that basing N applications on in-season monitoring can reduce N applications in low yielding seasons and match the yield potential in high-yielding seasons.